1. Field of the Invention
The present invention relates to a blade mounting device in a cutting tool such as a jig saw and a reciprocating s aw, and particularly to a blade mounting device having a holder mechanism which is operable to fix a blade to a drive shaft of the tool and to permit removal of the blade from the drive shaft as an actuation rod is rotated.
2. Description of the Prior Art
U.S. Pat. No. 5,165,173 discloses a blade mounting device as shown in FIG. 16 which corresponds to FIG. 5 of this patent. The blade mounting device includes a tubular drive shaft 103 having a female thread formed on Its inner peripheral surface. A lock screw or actuation rod 128 is in engagement with the female thread of the drive shaft 103, so that a blade B can be held in position or can be released by rotating the actuation rod 128 to move the same toward and away from the head of the blade B. The actuation rod 128 has an extension 129 having a semi-circular configuration in section and extending upwardly. An operation rod or connecting rod 130 having a semi-circular configuration in section similar to the sectional configuration of the extension 129 is inserted into the drive shaft 103 with its flat side surface slidably contacting the flat side surface of the extension 129. (The connecting rod 130 is not threadably engaged with the inner wall of the drive shaft 103.) An operation knob 137 is connected to the upper portion of the connecting rod 130 via a clutch 131, so that the extension 129 or the actuation rod 128 as well as the connecting rod 130 can be rotated when the operation knob 137 is rotated.
However, with this conventional construction, the rotational force applied to the operation knob 137 is transmitted to the actuation rod 128 by rotating the connecting rod 130 together with the extension 129 with the flat side surfaces of the connecting rod 130 and the extension 129 contacting each other. Since the connecting rod 130 and the extension 129 are inserted into the driving shaft 103 having a diameter which cannot be unduly increased from a viewpoint of design, they may not have a diameter which provides a sufficient strength. For this reason, when the operation knob 137 is rotated, the operation rod 130 or the extension 129 tends to be resiliently deformed and distorted. Otherwise, the operation rod 130 and the extension 129 may slide to be displaced from each other in the diametrical direction. When such distortion or displacement has been caused, the operational rod 130 or the extension 129 is pressed on the inner peripheral surface of the drive shaft 103, so that a resistance force against the rotation of the operational rod 130 or the extension 129 is produced to reduce the rotational force to be transmitted. Therefore, the rotational force transmitted to the extension 129 becomes unstable, and consequently, the blade B cannot be reliably fixed in position.